1. Field of Invention
The present invention relates to non-destructive measurements of the thickness of internal coatings for containers and of the exposed areas of the container not covered by the coatings and, in particular, the present invention relates to an apparatus and method for making a plurality of measurements of internal coating thicknesses and exposed metal areas in imperfections in the internal coatings for steel and aluminum beverage cans without destroying either the internal coating or the beverage can in the process of making the measurements.
2. Statement of the Problem
Internal coatings are provided for steel and aluminum beverage cans. These internal coatings have typical thicknesses in the range of about 0.1 to 0.5 mils. The coatings provide an inert layer between the substance placed in the container and the interior metal surface of the container so as to prevent, for example, corrosion. Such internal coatings are usually comprised of acrylic-based material sprayed or electrostatically deposited into the interior of a container at high speeds in an assembly line.
A need exists to provide an instrument and method for measuring the internal coating thickness of the container at various points within the container and to further measure the area of exposed metal found in areas of coating imperfections. Such measurements become complicated due to the different internal configurations a container may have especially in steel and aluminum beverage cans having curved neck and throat areas. These measurements are necessary to provide important feedback information for evaluating whether the coating application process is functioning properly.
One conventional approach is available for measuring the thickness of internal coatings. The STRANDGAUGE.TM. film measuring gauge manufactured by STRAND ELECTRONICS INC., 21175 Nunes Avenue, P.O. Box 2672, Castro Valley, Calif. 94546, is adapted to measure the thickness of many types of coating materials including most paints, varnishes and lacquers. The STRANDGAUGE.TM. instrument utilizes two types of probes: a flat plate probe for measuring thicknesses of coatings on flat surfaces and a formed can probe. The formed can probe utilizes two handles connected to a common fulcrum wherein the can is slipped over a single probe head. The handles are then released and a ground lead is clipped to another portion of the can to make contact with the metal surface. In the presence of any eye holes, scratches or other breaks in the film, the STRANDGAUGE.TM. instrument will fluctuate or go towards zero and in order to obtain a reading, the probe must be relocated to a different portion of the sample and a reading reattempted. The STRANDGAUGE.TM. instrument is not adaptable to read unusually configured internal areas such as the neck and throat areas of beverage cans. If a reading is attempted in these areas, the neck or throat area may become deformed and the coating cracked causing possible destruction in the coating. The STRANDGAUGE.TM. instrument is capable of only single readings and does not measure the area of exposed metal due to internal coating defects.
A need exists for an apparatus and method for the non-destructive measurement of the internal coating thicknesses and exposed metal areas in containers such as steel or aluminum beverage cans and for an apparatus that performs such measurements at a plurality of locations within the container including the throat and neck areas of a can. A need exists for an apparatus and method adaptable for use in a factory environment being capable of data storage, down loading of data, and displaying and/or printing out of the obtained data. A need further exists for an apparatus and method having suitable structural provisions incorporated so as not to damage the internal coating or the sensors upon insertion or release of the container being measured from the apparatus. A need further exists for an apparatus and method that allows repetitive readings in the same place in the container and the ability to create a profile of the internal coating of the container at precise locations within the can.
3. Solution to the Problem
The present invention provides a solution to the problem by providing a non-destructive apparatus and method for measuring a plurality of areas along a line of the container so that the throat, neck and sidewall areas of the container can be equally measured. Each measurement under the present invention is capable of obtaining the thickness of the coating or, if in the presence of a defect, the area of exposed metal.
The present invention, unlike the STRANDGAUGE.TM., provides two signals, a first output signal based upon the capacitive reactance which is indicative of the thickness of the internal coating and a second output signal based upon the resistive conductance which is indicative of the area of exposed metal in the internal coating.
The apparatus and method of the present invention is fully automatic and is adapted for factory and production line use being fully capable of storing data, down loading data, displaying and printing out the recorded data.
The present invention is designed not to damage the internal coating of the container or the sensors during the testing procedure through operator carelessness or the like.
Finally, the present invention is capable of repetitive readings in the same physical location on the coating and has the ability of creating profiles around various internal circumferences of the container.